The present invention is directed to a fuel-saving energy storage drive system for engine-driven vehicles of all types, and is particularly effective for those vehicles which operate primarily under stop-and-go driving conditions prevalent in urban areas or whose tasks are such that they involve repeated acceleration and deceleration regardless of the area of operation.
For many years there has been recognition that vehicles could be made more fuel-efficient if the energy normally lost in decelerating or braking the vehicle could be somehow collected, stored and reused to accelerate the vehicle. A relatively large number of prior patents and published patent applications exist which are directed to various aspects of this general approach.
Some workers in the field have sought to collect and store the energy in hydraulic accumulators and then reuse the energy through fixed or variable displacement hydraulic transmissions. The most pertinent examples of this approach may be found in British Pat. No. 955,604, U.S. Pat. Nos. 3,892,283, 3,903,696 and 4,098,083, and in Improvement of Citybus Fuel Economy Using a Hydraulic Hybrid Propulsion System--A Theoretical and Experimental Study, SAE Technical Paper 790305 (1979).
Certain variations of this concept which utilize a flywheel as an accumulator for collecting and storing vehicle deceleration energy, either alone or in combination with a hydraulic accumulator, are shown in the following U.S. Pat. Nos.: 3,665,788; 4,018,052; 4,037,409; 4,098,144; and 4,110,982. Also significant in this regard are published German patent application Nos: 24 51 021; 24 54 753; 24 62 058; 25 15 048; 25 51 580; 26 37 322; 26 41 886; 26 49 241; 27 54 623 and 28 10 086.
The foregoing examples of work in the field to date have been concerned primarily only with the very basic mechanical aspects of developing a workable automotive transmission wherein the collection, storage and reuse or regeneration of braking energy is feasible, the thought being that any workable transmission which is capable of regenerating braking energy will save fuel. On the other hand, little thought has as yet been given to particular methods of control of the regenerative system so as to maximize its efficiency and thereby maximize the amount of energy which can be saved by its use.
Moreover, the emphasis on the importance of the brake energy regeneration capabilities of the transmission has tended to suppress the emphasis on the possible significance of other factors usable, either alone or in combination with a regenerative transmission, which by themselves can produce additional energy and fuel savings perhaps equal to those produced by brake energy regeneration alone. A principal factor to be considered in this regard is the efficient operation of the vehicle engine. Previously the recognition of the significance of this factor in combination with an energy storage transmission was exhibited primarily by the work of Vincent E. Carman as described by Thomas, A Car That Can Store Power, 73 Mechanix Illustrated 60, 62 (1977) in which a hydraulic brake energy regenerative transmission is described wherein a hydraulic accumulator stores energy supplied both from deceleration of the vehicle and from an engine-driven pump. When pressure in the accumulator reaches a predetermined point, the engine is switched off automatically and, when the accumulator pressure drops below a predetermined point, the engine is automatically started to drive the pump and increase accumulator pressure. In this system the engine does not idle, and is completely stopped for extended periods of time while the vehicle is in motion, thereby saving the fuel which would otherwise be wasted by engine idling. Moreover, when the engine is running, it is not driving the car directly but rather is pumping energy into the accumulator, thereby permitting the engine to be governed at a constant and relatively efficient speed. This type of engine control, whether used alone or in conjunction with a regenerative transmission, can produce an additional energy savings approximately equal to that produced by a regenerative system alone.
However, just as with the regenerative system, different efficiencies are obtainable depending upon the particular manner in which an engine stop-start system, and an engine governing system, are operated. For example, making engine starting dependent upon a fixed predetermined accumulator pressure does not take into account the variable ability of the vehicle regenerative system to supply energy to the accumulator when decelerating from variable vehicle speeds. Allowing the engine-driven pump to charge the accumulator to a relatively high pressure may be appropriate from an efficiency point of view when the vehicle is motionless, but it would not be appropriate to use engine energy to charge the accumulator to the same high pressure when the vehicle is in motion, and particularly not so when the speed of the vehicle is relatively high because at such speed the vehicle has an increased potential for supplying energy to the accumulator from deceleration. If the accumulator is already charged too highly by the engine it will not be able to accept energy supplied to it from deceleration, and such energy will therefore be wasted.
Moreover, governing of engine speed by itself does not produce as energy-efficient an operation as when both engine speed and engine output torque are governed. In addition, too frequent stopping and starting of the engine to charge the accumulator may be harmful to efficiency because of excessive use of energy to start the engine, and also be harmful to the acceptability of the vehicle's exhaust emissions.
There are widely varying degrees of efficiency with which stored energy, whether stored from vehicle deceleration or from the engine or from both, can be used to drive a vehicle. It will not always be most efficient to drive a vehicle from stored energy; under certain conditions it will be more efficient to drive the vehicle directly by means of the engine, and in such case engine speed should not be governed but should be controllable in response to operator torque demands.
Furthermore, when using or collecting the stored energy, significant wastage can occur if steps are not taken to prevent inefficient use of the transmission, particularly of its variable displacement hydraulic components, and to prevent wheel skidding and wheel spinning. The latter can pose a particular problem due to the high level of torque which can be produced from an energy storage transmission despite the reduced size of the engine.
In addition, to make fuel-efficient drive systems of the type described available in large quantity so as to use their advantages to combat the current energy crisis at the earliest possible date, they must not require extensive redesign of existing vehicles but should, preferably, be capable of installation in conventional vehicle drive trains as replacements for conventional transmissions now being used, with a minimum of complexity and structural modification to the vehicle.